This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A poly(A) tail is found at the 3'ends of nearly every mRNA and many primary microRNA transcripts, and it influences many aspects of RNA metabolism. Although many essential mammalian 3'processing factors have been identified through conventional biochemical analysis, the full protein composition of the 3'processing complex has not been characterized, and the regulation of this process is still poorly understood. In this study, we isolated highly purified human 3'processing complexes and determined their protein composition by using multidimensional protein identification technology (MudPIT). In addition to known 3'processing factors, our complex contained homologues of yeast 3'processing factors, and proteins that may link 3'processing to transcription, chromatin remodeling, splicing, RNA quality control, export, translation, and possibly Ca2+ signaling. Most surprisingly, many factors with functions in DNA repair were identified. This observation reveals an unexpected similarity between 3'processing and transcription, in which several DNA repair factors are known to play essential roles. Finally, we validated the involvement of several factors identified in this study in 3'processing.